Today various services are provided using radio communications, and the demand for allocating new frequencies for new services is constantly increasing. However radio frequencies are a limited shared resource, and in order to efficiently use frequencies, the spacing of each frequency band in use is becoming closer and closer.
In the radio communication for portable telephones, an orthogonal modulation system for a modulating carrier wave by an input signal comprised of an in-phase signal component and an orthogonal signal component is used as a modulation method for transmission signals. In the case of an orthogonal modulation system, carrier leaking components are generated in the modulated waves, and if the amount of carrier leaking components is high, the error rate of reproduced data in demodulation at the receive side increases, which causes a quality deterioration of other radio communication lines. Therefore carrier leaking components must be suppressed. In order to suppress carrier leaking in the output of the modulator, the DC component is decreased to zero in the orthogonal modulator. A conventional way of adjusting carrier leaking is to adjust an input signal comprised of an in-phase signal component and an orthogonal signal component so that the DC component in the base band input of the orthogonal modulator becomes zero.
FIG. 1 is a diagram depicting a configuration example of a conventional transmission device. Each input signal comprised of an in-phase signal component (I component) and orthogonal signal component (Q component) is input to an orthogonal modulator 14 by way of an adder 10,11 and a D/A converter 12, 13 respectively. The orthogonal modulator 14 modulates a carrier wave from a local oscillator 15 based on these signals, and outputs a transmission modulation signal. The transmission modulation signal which is output from the orthogonal modulator 14 is amplified by an amplifier 16, and is output. A part of the transmission modulation signal is fetched by a directional coupler 17, and is converted into a demodulation signal by a demodulation unit 18. A DC offset computing unit 19 detects a carrier leaking component from the demodulation signal, and inputs a DC offset value, whereby the carrier leaking component included in the transmission modulation signal is the minimum, to the adder 10, 11. The adder 10, 11 adds the DC offset value to each input signal.
The configuration of the orthogonal modulator which compensates the carrier leaking component using the DC offset value is disclosed in Patent Documents 1 and 2, for example.    Patent Document 1 Japanese Patent Application Laid-Open NO. H5-22356    Patent Document 2 Japanese Patent Application Laid-Open NO. No. 2001-339452